1. Field of the Invention
The present invention relates to driving circuits and driving methods for liquid crystal display devices, and more specifically, to driving circuits and driving methods for driving auxiliary capacitance electrodes.
2. Description of the Related Art
Active matrix liquid crystal display devices provided with TFTs (thin film transistors) as switching elements have been known for several years. Such liquid crystal display devices are provided with a liquid crystal panel made of two insulating substrates that are arranged opposite one another. On one substrate of the liquid crystal panel, scanning signal lines (gate bus lines) and video signal lines (source bus lines) are arranged in a lattice, and TFTs are arranged near the intersections of the scanning signal lines and the video signal lines. Each of the TFTs is made of a gate electrode branching off from the scanning signal lines, a source electrode branching off from the video signal lines, and a drain electrode. The drain electrodes are connected to pixel electrodes that are arranged in a matrix on the substrate for forming an image. Also, the substrate on the other side of the liquid crystal panel is provided with an electrode (referred to as “common electrode” below) for applying a voltage between the pixel electrodes and the common electrode. Liquid crystal capacitances are formed by the pixel electrodes and the common electrode.
With such a liquid crystal display device, in order to sequentially select each of the scanning signal lines for one horizontal scanning period each, the application of an active scanning signal to each of the scanning signal lines is repeated with a cycle of one vertical scanning period. Therefore, the charges that have accumulated in the liquid crystal capacitances have to be held for about one vertical scanning period. Since the accumulated charges cannot be held with the liquid crystal capacitances alone, auxiliary capacitances are provided in parallel to the liquid crystal capacitances.
FIG. 5 is a circuit diagram showing the vicinity of a TFT 51 of a conventional liquid crystal display device. A gate electrode 57 of the TFT 51 is connected to a scanning signal line GL, its source electrode 58 is connected to a video signal line SL, and its drain electrode 59 is connected to a pixel electrode 52. Also, a liquid crystal capacitance 55 and an auxiliary capacitance 56 are arranged in parallel. The liquid crystal capacitance 55 is formed by the pixel electrode 52 and the common electrode 53, whereas the auxiliary capacitance 56 is formed by the pixel electrode 52 and the auxiliary capacitance electrode 54.
In a liquid crystal display device as described above, the auxiliary capacitance electrode 54 is arranged on the same substrate as the pixel electrode 52, and there is the possibility of a leakage current, caused by manufacturing defects or the like, flowing between the pixel electrode 52 and the auxiliary capacitance electrode 54. The effect that a leakage current flowing between the pixel electrode 52 and the auxiliary capacitance electrode 54 has is explained with reference to FIGS. 5 and 6. In the configuration shown in FIG. 5, if there is a leakage current flowing between the pixel electrode 52 and the auxiliary capacitance electrode 54, then the liquid crystal display device operates as if there is a short-circuit, as shown in FIG. 6, and the potential of the pixel electrode 52 becomes equal to the potential of the auxiliary capacitance electrode 54. As a result, a voltage corresponding to the potential difference between the auxiliary capacitance electrode 54 and the common electrode 53 is applied to the liquid crystal capacitance 55. Conventionally, the liquid crystal display device is driven such that the potential of the auxiliary capacitance electrode 54 (referred to as “auxiliary capacitance electrode potential”) is the same as the potential of the common electrode 53 (referred to as “common electrode potential”). Therefore, in a liquid crystal display device operated in normally-white mode, for example, the locations where the above-mentioned leakage currents occur can be perceived as bright dots. These are also known as bright dot defects, and conventionally, measures to let the bright dot defects be displayed in black (blackening) are performed with a laser or the like.
To address this problem, JP 2001-188217A discloses a liquid crystal display device in which the auxiliary capacitance electrode 54 and the common electrode 53 are driven such that there is always a predetermined potential difference between the auxiliary capacitance electrode potential and the common electrode potential. FIG. 7 is a block diagram showing the overall configuration of such a liquid crystal display device. This liquid crystal display device comprises a gate power source 100, a display control circuit 200, a video signal line driving circuit 300, a scanning signal line driving circuit 400, a liquid crystal panel 500, a common electrode driving circuit 600, an auxiliary capacitance electrode driving circuit 700, and an auxiliary capacitance potential setting circuit 800. Inside the liquid crystal panel 500, a plurality of scanning signal lines GL and a plurality of video signal lines SL are arranged in a lattice, and TFTs 51 serving as switching elements are arranged at the intersections between the scanning signal lines GL and the video signal lines SL. The gate electrodes 57 of the TFTs 51 are connected to the scanning signal lines GL, its source electrodes 58 are connected to the video signal lines SL, and its drain electrodes 59 are connected to the pixel electrodes 52. The common electrode 53 is arranged in opposition to the pixel electrodes 52, and liquid crystal capacitances 55 are formed by the pixel electrodes 52 and the common electrode 53. Moreover, auxiliary capacitance electrodes 54 are provided on the substrate on which the pixel electrodes 52 are provided, and auxiliary capacitances 56 are formed by the pixel electrodes 52 and the auxiliary capacitance electrodes 54. The scanning signal lines GL are connected to the scanning signal line driving circuit 400, and the video signal lines SL are connected to the video signal line driving circuit 300. The auxiliary capacitance electrodes 54 are connected to an auxiliary capacitance electrode driving signal line CSL, and the auxiliary capacitance electrode driving signal line CSL is connected to the auxiliary capacitance electrode driving circuit 700. It should be noted that for the sake of convenience only a portion of the internal configuration of the liquid crystal panel 500 is shown.
An auxiliary capacitance electrode driving signal for driving the auxiliary capacitance electrodes 54 is outputted by the auxiliary capacitance electrode driving circuit 700. An upper limit for the auxiliary capacitance electrode potential applied by the auxiliary capacitance electrode driving signal to the auxiliary capacitance electrodes 54 is set by the auxiliary capacitance potential setting circuit 800, as explained below. As shown in FIG. 7, the auxiliary capacitance potential setting circuit 800 includes a diode 81, resistors 83 and 84, and a capacitor 85. The anode of the diode 81 is connected to the auxiliary capacitance electrode driving signal line CSL. On the other hand, the cathode of the diode 81 is connected to a gate-off power source line GOFFL. Also, one terminal of the gate-off power source line GOFFL is connected to a ground conductor 82. The node 89 connecting the anode of the diode 81 and the auxiliary capacitance electrode driving signal line CSL is connected via a capacitor 85 to the auxiliary capacitance electrode driving circuit 700. With this configuration, the auxiliary capacitance potential setting circuit 800 functions as a so-called clamping circuit. Note that the gate-off power source line GOFFL is a power source line for supplying the voltage with the lower potential of the voltages to be supplied from the gate power source 100 to the scanning signal line driving circuit 400. The voltage with the lower potential of the voltages to be supplied from the gate power source 100 to the scanning signal line driving circuit 400 is referred to below as “gate-off voltage”. The voltage with the higher potential of the voltages to be supplied from the gate power source 100 to the scanning signal line driving circuit 400 is referred to below as “gate-on voltage”.
Next, the driving of the auxiliary capacitance electrodes 54 is described with reference to FIGS. 7, 8 and 9. The node 89 connecting the anode of the diode 81 and the auxiliary capacitance electrode driving signal line CSL is connected to one side of the capacitor 85. FIG. 8 is a waveform diagram showing the change of the potential of the node 90 between the other side of the capacitor 85 and the auxiliary capacitance electrode driving circuit 700. FIG. 9 is a waveform diagram showing the change of the potential at the node 89. In FIG. 8 and FIG. 9, the potential at the node 88 between the cathode of the diode 81 and the gate-off power source line GOFFL is denoted as Va, the potential at the node 89 is denoted as Vb, and the potential at the node 90 is denoted as Vd. Also, the potential difference (Vd−Va) between the potential Vd and the potential Va when the potential Vd at the node 90 is at high potential is denoted as Vc, whereas the potential difference (Va−Vd) between the potential Vd and the potential Va when the potential Vd at the node 90 is at low potential is denoted as Ve. An auxiliary capacitance electrode driving signal in which high potential and low potential alternate every horizontal scanning period (1H) is outputted from the auxiliary capacitance electrode driving circuit 700, and the potential Vd changes as shown in FIG. 8. When the potential Vd is lower than the potential Va, then the potential Vb increases together with an increase in the potential Vd until the potential Vb and the potential Va are the same potential. At this time, the diode 81 is non-conducting. When the potential Vd is further increased, the diode 81 becomes conducting after the potential Vb and the potential Va have become the same potential. The capacitor 85 is charged in accordance with the potential difference between the potential Vd and the potential Va. The potential Vb does not become higher than the potential Va. When the potential Vd falls, also the potential Vb falls accordingly. At this time, the potential Vb becomes lower than the potential Va by a potential difference corresponding to the sum of the potential difference (Va−Vd) between the potential Vd and the potential Va and the potential difference due to the charging of the capacitor 85. As a result, the potential Vb changes as shown in FIG. 9. Here, the node 89 and the auxiliary capacitance electrodes 54 are at the same potential, so that also the auxiliary capacitance electrode potential Vcs changes as shown in FIG. 9. On the other hand, also the common electrode potential Vcom changes such that high potential and low potential alternate every single horizontal scanning period (1H), but its lower limit is substantially equal to the ground potential GND. Thus, the auxiliary capacitance electrode potential Vcs and the common electrode potential Vcom change as shown in FIG. 10. Note that there is a voltage drop when the diode 81 is conducting, but since this voltage drop is sufficiently small, it can be ignored for the purposes of this explanation.
As shown in FIG. 10, there is always a predetermined potential difference between the auxiliary capacitance electrode potential Vcs and the common electrode potential Vcom. When there is a leakage current, then a voltage corresponding to the potential difference between the auxiliary capacitance electrode potential Vcs and the common electrode potential Vcom is applied to the liquid crystal capacitance 55, as described above. Thus, in the case of liquid crystal display devices employing the normally-white mode, if the auxiliary capacitance electrode potential Vcs and the common electrode potential Vcom are at the same potential, then leakage currents become a cause of bright dots, but if a predetermined potential difference is always maintained between the auxiliary capacitance electrode potential Vcs and the common electrode potential Vcom, then it can be ensured that the pixels where leakage currents occur are displayed in black.
Now, with the above-described liquid crystal display device, the upper limit of the auxiliary capacitance electrode potential Vcs is set based on a voltage supplied with a power source line, such as the gate-off power source line GOFFL. Therefore, if the gate-off power source line GOFFL is used in this manner to set the upper limit of the auxiliary capacitance electrode potential Vcs, then the gate power source 100 requires a power source capability that can supply a voltage for setting the upper limit of the auxiliary capacitor electrode voltage Vcs in addition to the gate-on voltage and the gate-off voltage supplied to the scanning signal line driving circuit 400. There are furthermore the costs required for the parts constituting the auxiliary capacitance potential setting circuit 800 shown in FIG. 7.